In the production of crude oil, it is frequently found that the crude oil is sufficiently viscous to require the injection of steam into the petroleum reservoir. Ideally, the petroleum reservoir would be completely homogeneous and the steam would enter all portions of the reservoir evenly. However, it is often found that this does not occur. Instead, steam selectively enters a small portion of the reservoir while effectively bypassing other portions of the reservoir. Eventually, "steam breakthrough" occurs and most of the steam flows directly from an injection well to a production well, bypassing a large part of the petroleum reservoir.
It is possible to overcome this problem with various remedial measures, e.g., by plugging off certain portions of the injection well. For example, see U.S. Pat. Nos. 4,470,462 and 4,501,329, assigned to the assignee of the present invention. However, to institute these remedial measures, it is necessary to determine which portions of the reservoir are selectively receiving the injected steam. This is often a difficult problem.
Various methods have been proposed for determining how injected steam is being distributed in the wellbore. Bookout ("Injection Profiles During Steam Injection", SPE Paper No. 801-43C, May 3, 1967) summarizes the known methods for determining steam injection profiles and is incorporated herein by reference for all purposes.
The first and most widely used of these methods is known as a "spinner survey". A tool containing a freely rotating impeller is placed in the wellbore. As steam passes the impeller, it rotates at a rate which depends on the velocity of the steam. The rotation of the impeller is translated into an electrical signal which is transmitted up the logging cable to the surface where it is recorded on a strip chart or other recording device.
As is well known to those skilled in the art, these spinners are greatly affected by the quality of the steam injected into the well, leading to unreliable results or results which cannot be interpreted in any way.
Radioactive tracer surveys are also used in many situations. With this method methyl iodide (131) has been used to trace the vapor phase. Sodium iodide has been used to trace the liquid phase. The radioactive Iodine is injected into the steam between the injection well and the steam generator. The tracer moves down the tubing with the steam until it reaches the formation, where the tracer is temporarily held on the face of the formation for several minutes. A typical gamma ray log is then run immediately following the tracer injection. The recorded gamma ray intensity at any point in the well is then assumed to be proportional to the amount of steam injected at that point.
The vapor phase tracers have variously been described as alkyl halides (methyl iodide, methyl bromide, and ethyl bromide) or elemental iodine. It has been found that the above materials undergo chemical reactions that dramatically affect the accuracy of the results of the survey.
It is, therefore, desirable to devise a highly accurate method of developing steam profiles in steam injection wells.